1. Field of the Invention
This invention relates to a magnetic memory for storing information in a magneto-resistivity effect element.
2. Description of the Related Art
In recent years, the MRAM (Magnetic Random Access Memory) has been attracting attention as a memory cell for use in such information processing devices as computers and communication facilities. Since the MRAM is capable of memorizing data magnetically and therefore maintaining directions of magnetization without having to use any electric means, it is safe from incurring the disadvantage of losing information due to power failure as experienced by the DRAM (Dynamic Random Access Memory) and SRAM (Static RAM) which are volatile memories. The MRAM also excels such conventional nonvolatile memories as the flash EEPROM and the hard disk device in terms of access speed, reliability, and power consumption. The MRAM, therefore, is reputed to be capable of alternating all the functions of such volatile memories as DRAM and SRAM and the functions of such nonvolatile memories as the flash EEPROM and the hard disc devices.
In the development of an information device directed toward the so-called ubiquitous computing which enables information to be processed everywhere at the same time, the desirability of such a memory which fulfills high-speed processing and decreases power consumption as well and permits avoidance of loss of information even in the case of power failure finds general recognition. The MRAM promises to satisfy all these requirements and is expected to find adoption in numerous information devices in the future.
Particularly, cards and portable information terminals which are carried daily by users on their persons often fail to secure sufficient power supply. When a large volume of information is processed under a harsh environment of utility, therefore, even the MRAM which boasts of low power consumption is required to allow further decrease of power consumption during the course of processing information.
As one example of the technique which is capable of further decreasing power consumption in the MRAM, the magnetic memory which is published on page 133 of the Nov. 18, 2002 issue of the “Nikkei Electronics” is now available. This magnetic memory 500, as illustrated in FIG. 19, is provided in each of the individual memory regions (memory cells) thereof with a bit line 502, a word line 504 disposed perpendicularly to the bit line 502, a tunneling magneto-resistive element (TMR) 506 disposed between the bit line 502 and the word line at the position of intersection thereof, and a transistor 508 connected to the TMR element. The bit line 502 and the word line 506 are each adapted to generate nearly half of the magnetic field necessary for reversing the bit state (magnetized state) of the TMR element 506. When an electric current is advanced through the bit line 502 and word line 504 which have been selected, the magnetized state of the TMR element 506 is suitably inverted at the point of intersection of these lines.
This magnetic memory 500 is so configured that both the bit lines 502 and the word lines 504 are furnished with a structure clad with a ferromagnetic thin film 510 of high permeability. Thus, the leaks of magnetic flux from the bit line 502 and the word line 504 can be reduced. During the passage of electric current through the bit lines 502 and the word lines 504, since the ferromagnetic thin film 510 is made to generate a static magnetic field in consequence of the magnetization of the ferromagnetic thin film 510, the sum of this static magnetic field of the ferromagnetic thin film 510 and the induction magnetic fields of the bit lines 502 and the word lines 504 is applied to the TMR element 500. As a result, the magnetic field necessary for the inversion of the magnetized state of the TMR element 506 can be obtained even when the state of power consumption is low.
Since this magnetic memory is enabled to concentrate the magnetic flux in the TMR element 506 by keeping the bit lines and the word lines covered each on three surfaces with the ferromagnetic thin film 510 and leaving their surfaces on the TMR element 506 side open, it enjoys an advantage of reducing the writing time.
Incidentally, the TMR element is furnished with a first magnetizing layer (magnetic susceptibility layer) enabled to change the direction of magnetization by an external magnetic field, a second magnetic layer having a fixed direction of magnetization, and a non-magnetic insulating layer interposed between the first magnetic layer and the second magnetic layer and is adapted to memorizes binary data by controlling the direction of magnetization of the first magnetic layer parallel or not parallel to the direction of magnetization of the second magnetic layer.
A further study made by the present inventor, however, has ascertained that the coating of the bit lines 501 and the word lines 504 with the ferromagnetic thin film 510 is liable to induce dispersion in the produced magnetic field while it is indeed capable of reducing the magnitude of the electric current during the course of writing. Specifically, the ferromagnetic thin film 510 incurs difficulty in uniformly coating the bit lines 502 and the word lines along the directions of length thereof, eventually acquires in the interior thereof various directions of magnetization owing to the spontaneous formation of a plurality of magnetic domains. Because of these factors, the ferromagnetic thin film 510 entails the possibility of dispersing the magnetizing property to be imparted to the individual TMR elements 506 during the course of writing.
When the magnetic field is inverted by switching the directions of passage of electricity to the bit lines 502 and the word lines 504, the presence of the ferromagnetic thin film 510 results in posing the problem of rendering ununiform the speed of change and the intensity of the magnetic field between the electric current in the forward direction and the electric current in the reverse direction. As a result, the individual TMR elements 506 incur dispersion in the writing speed in one direction and the writing speed in the other direction and consequently cause an anxiety about complicating the control of electric current and the control of timing in the course of writing.
When the ferromagnetic thin film 501 happens to form numerous magnetic domains, it suffers emission of a Barkhausen noise during the change of the state of magnetization of the bit lines 502 and the word lines 504. This noise is considered to constitute a factor for deteriorating the writing property.
This invention, originated in the light of the problems enumerated above, is directed toward abating the dispersion of the magnetic field for writing in the magnetic memory and enhancing the writing property.